MAURICE GREENE came under further pressure last night to explain allegations that he bought banned performance-enhancing drugs, after a letter - widely believed to have been written by Ato Boldon, his former training partner and best friend - slammed the former 100 metres world-record holder and his coach, John Smith.

The author, clearly a sprinter, deals furiously with allegations by Angel Guillermo Heredia, a Mexican former discus thrower, that Greene gave him up to $40,000 for advice, steroid creams, EPO, insulin and stimulants in 2003 and 2004. The letter, which Observer Sport has seen, is addressed to Smith, Greene and Emmanuel Hudson, who used to represent Greene and Boldon for the management company HSI International. It accuses them of betraying the author by obtaining banned drugs behind his back, lying about competing clean and leaving a stain on his own achievements.

Boldon finished second to Greene in the 100m at the 2000 Olympics in Sydney and was world 200m champion three years earlier. He retired in 2004 and is now a coach and television commentator.

The letter says: 'My own coach, doping my competition while he smiles in my face and preaches the "we are clean and they are not" gospel.'

Greene has always denied being involved in doping, but admits he paid for supplies from Heredia, who is the US Government's main witness in a case against Marion Jones' former coach Trevor Graham that is due to start in San Francisco on 19 May. Greene claimed that it was for other members of his training group.

But the letter claims that the allegations, first published in the New York Times last Sunday, 'now taint everyone who has worked with you, even if they decided not to go this DRUG route. Say what you want about me and my failure to win the BIG ones, but I did it cleanly, and I can look you or anyone else in the face, not to mention myself in the mirror, for the rest of my life. I know that, and so DO YOU... John Smith the great "sprint guru" is nothing but the emperor with no clothes.'

The letter will also further embarrass the IAAF, who last month appointed Greene as an ambassador following his retirement in February. The sport's governing body have already asked him for his comments on the recent reports.